Modifications such as the one you did are not ticketed right away but go to a ticketing team, in a queue. They might reach the top of the queue before you travel, or might not. There is no point in calling, this will not accelerate the process.
If 48 hours before travel you are not ticketed, then call CS. If you are not ticketed the day of travel, arrive well in advance at the airport with a paper containing BA CS numbers, for the countries that will happen to have an open office at the time you are there.
This is a known quirk of BA, the airline who can not ticket properly.